Method for preparing particulate zinc oxide shapes of high surface area and improved strength

ABSTRACT

Particulate zinc oxide shaped of high surface area and improved crushing strength are prepared without extraneous binder by a process including forming normal surface area zinc oxide into shapes without binder, converting at least a portion of the zinc oxide to basic zinc carbonate by exposing the shapes in the presence of moisture, to a carbon dioxide-containing atmosphere and effecting substantially complete decomposition of the basic zinc carbonate. Conditions of bulk density, moisture content and temperature are controlled.

BACKGROUND OF THE INVENTION

Catalysts are employed in a variety of chemical processes such assynthesis and reforming of hydrocarbons, ammonia synthesis, etc. Manysuch catalysts consist of or contain metallic, i.e., nickel, iron,copper, chromium, etc., compounds which may be either temporarilydeactivated or permanently poisoned by certain contaminants orcomponents in a process stream. Prominent among the permanent poisonsare hydrogen sulfide and various organic sulfur compounds. With suchcatalysts it is essential that the sulfur be removed from the processstream to prevent contact with such catalysts.

Various methods for removing sulfur have been proposed and several arein current use including washing with sulfuric acid, adsorption onactivated carbon and absorption or chemisorption by various materials.With high-sulfur feedstocks, a combination of desulfurizing methods issometimes employed.

A material found to be particularly effective in reducing sulfur inprocess streams to an acceptable level, is zinc oxide. In U.S. Pat. No.1,868,096 the process disclosed includes passing a stream of water gasover one or more masses composed of zinc oxide or zinc oxide andchromium oxide before contact with a methanol catalyst which issensitive to sulfur poisoning. When used as a desulfurant, zinc oxide isconverted to zinc sulfide. The active life of a charge of zinc oxide ina desulfurizing reactor is obviously dependent upon the sulfur contentof the feedstock being processed. Nevertheless, maximum utilization ofthe zinc oxide itself is also of importance. In that regard, theavailability of the zinc oxide for the desulfurizing reaction and thequantity of zinc oxide that can be contained within an available reactorvolume have important effects upon the active life of the zinc oxidecharge.

The amount of normal zinc oxide that can be contained in a given volumedepends upon its purity and its bulk density. A product of low purityhas an inherent disadvantage in that inactive impurities will occupy aportion of the reactor volume in more or less direct proportion to theirpercentage in the zinc oxide product. Hence, a normal zinc oxide of highpurity is most desirable. The weight that can be charged into a reactoris directly related to its bulk density, that is, the weight of a givenvolume of product. Light, fluffy zinc oxides are at a stoichiometricdisadvantage in that relatively small weights of such products willoccupy the available reactor volume.

The availability of the zinc oxide for reaction with hydrogen sulfideand other sulfur-containing compounds is a function of its specificsurface area, that is, the surface area of one gram of solid; at times,this is referred to simply as surface area. It is known that solids inthe form of fine particles exhibit larger specific surface areas andgreater reactivity than those of greater particle size and that solidscan be produced in an "active" or highly dispersed state by thermaldecomposition of a parent substance from which a volatile component isreleased. In U.S. Pat. No. 1,878,390 relating to the preparation ofcatalysts suitable for methanol production, it is disclosed thatcatalysts comprising zinc oxide or a mixture of zinc oxide and chromiumoxide are particularly effective when prepared by heating the basiccarbonates. The preparation of a highly reactive zinc oxide ofexceedingly small particle size by the heating of ammoniated zinccarbonate is disclosed in U.S. Pat. No. 2,898,191 and Russian Pat. No.308,976. The advantages of controlling the surface area of zinc oxidefor use in removing sulfur compounds from industrial gas streams whereinsteam is admixed is disclosed in U.S. Pat. No. 3,441,370; zinc oxideshaving surface areas above 30 square meters per gram are said to be ofparticular advantage. The preparation of these high surface area zincoxides is by a wet process including precipitation of zinc carbonatefollowed by calcination for conversion to zinc oxide.

Although high specific surface area is a prime requisite of the zincoxide intended for use in desulfurizing industrial gas streams, it isequally important that the zinc oxide be in a form suitable for charginginto reactor towers. The physical form should be such that adequateresistance to handling, rather rough at times as in shipping, isprovided. High bulk density is also desired in order that an adequateweight can be packed into a reasonable volume. Since the latter is atvariance with the requirement of high surface area for availability andreactivity, in that exceedingly finely divided solids are usually of lowbulk density, agglomeration of high surface area products into a formsuitable for charging into desulfurizing reactors is required. Variousmethods of agglomeration have been proposed including forming a pasteand extruding it into cylinders which can be cut to desired lengths,forming tablets in a press, forming spherical pellets either in rotatingdrums or on rotating discs, etc. Normally an appreciable proportion of abinding material is required for adequate strength, particularly withhigh surface area zinc oxide, whereby the zinc oxide content isproportionately reduced. Optimum life of a zinc oxide product intendedfor use in desulfurizing industrial gas streams requires considerationof all of these factors.

SUMMARY OF THE INVENTION

It has now been discovered that particulate zinc oxide shapes withexcellent physical properties and a high capacity for absorption ofhydrogen sulfide can be prepared without a binder by a process whichincludes the steps of forming normal zinc oxide having a normal surfacearea into shapes of a size dependent upon the requirements of theintended use, effecting conversion of at least a portion of the normalzinc oxide to basic zinc carbonate (5ZnO·2CO₂ ·3H₂ O) by exposure of thenormal zinc oxide shapes in the presence of moisture to an atmospherecontaining carbon dioxide and subsequently effecting decomposition ofthe basic zinc carbonate to zinc oxide.

The results achieved by this process are quite unexpected in that notonly is the surface area of the normal zinc oxide increased but thestrength and hardness of these shapes, produced without extraneousbinder of any kind, surpass those of most configurations of similar highsurface area zinc oxide produced with a binder. Additionally, the shapesproduced by this process do not dust, an important consideration sincegas flow, through a bed of zinc oxide which dusts, can be seriouslyimpeded. The product of the present invention has also been found to bemore effective in removing sulfur than presently available commercialproducts.

PREFERRED EMBODIMENT

A normal zinc oxide having a specific surface area of less than about 10square meters per gram is first formed into shapes. The size of theseshapes is determined by the equipment in which they will be used. Thezinc oxide shapes may have a moisture content of up to about 30% H₂ Opreferably about 20 to about 25% H₂ O. The zinc oxide shapes are thenexposed to an atmosphere containing about 40 to 100% CO₂ and about 0 toabout 60% H₂ O at a temperature of about 50° to about 90° C preferablyabout 90 to about 100% CO₂, about 0 to about 10% H₂ O and about 60° to75° to 75° C. The shapes are exposed to these conditions for up to about4 hours. The resulting shapes contain at least about 30% basic zinccarbonate (5ZnO·2CO₂ ·3H₂ O) and preferably at least about 55% basiczinc carbonate. The basic zinc carbonate shapes are then heated in theambient atmosphere at a temperature of about 200° to 500° C to effectsubstantially complete decomposition to zinc oxide. Temperatures ofabout 250° to 350° C are preferred. The decomposition of basic zinccarbonate to zinc oxide is endothermic. The rate at which heat issupplied will therefore affect the time required to complete thereaction. In general, about 1 to about 5 hours are required.

The zinc oxide content of the thus treated zinc oxide shapes dependsalmost entirely on the purity of the starting normal zinc oxide in thatthe addition of extraneous binding agents is not required. With normalzinc oxides, entirely satisfactory starting materials, the resultingshapes contain at least about 95% zinc oxide, preferably in excess of98% zinc oxide. These treated shapes have a specific surface areagreater than about 30 square meters per gram. The crush strength of thetreated shapes is about 5 to about 30 times greater than that of theuntreated shapes and generally at least about 1500 grams.

The increase in the surface area of the zinc oxide resulting from theprocess of the present invention depends primarily upon the extent towhich the starting zinc oxide is converted to basic zinc carbonate.Surface areas are determined by methods including measurement of thequantity of a gas that is adsorbed on the surface of a sample andcalculation of the total surface area of the particular sample from theadsorption data obtained. Samples may contain particles of various sizesand various surface areas, therefore, the values obtained represent anaverage. The effect of conversion, from normal zinc oxide to basic zinccarbonate, on surface area will be seen in Table I.

                  TABLE I                                                         ______________________________________                                        % Conversion    Surface Area**                                                Normal ZnO to BZC*                                                                            (square meters per gram)                                      ______________________________________                                        76              50                                                            74              46                                                            66              43                                                            50              38                                                            36              27                                                            21              14                                                            0               4                                                             ______________________________________                                         *BZC = Basic Zinc Carbonate.                                                  **Surface area of carbonated product after heating at 350° C for 1     hour to effect decomposition to zinc oxide.                              

As indicated previously, the surface area values represent an averagefor a particular sample. In the case of the product of the presentinvention, the surface area of zinc oxide which is not converted tobasic zinc carbonate will remain substantially unchanged, while thesurface area of the normal zinc oxide that is converted to basic zinccarbonate will be increased manyfold. In order to obtain products havingsurface areas greater than about 30 square meters per gram (sq.m./gm.),conversions of zinc oxide to basic zinc carbonate of at least about 45%are required. Similarly, as shown in Table I in order to obtain asurface area of about 50 square meters per gram (sq. m./gm.), conversionof at least about 76% normal zinc oxide to basic zinc carbonate isrequired. However as will be seen later, the surface area of the zincoxide shapes also depends to a great extent upon the conditions underwhich the basic zinc carbonate is decomposed to zinc oxide.

The extent to which the normal zinc oxide can be converted to basic zinccarbonate depends upon the structure and bulk density of the startingnormal zinc oxide shapes subjected to carbonation as well as theconditions under which the carbonation reaction is effected. Thestructure of the shapes should be such that adequate permeability to thecarbon dioxide-containing atmosphere is provided. In that regard,compression or densification, particularly of the surface of the shapes,resulting in outer layers or shells of low porosity should be avoided.The bulk density of the starting shapes is also of importance in thatthe degree of conversion of normal zinc oxide to basic zinc carbonatevaries inversely with the bulk density of the normal zinc oxide shapes.In Table II, the bulk density of normal zinc oxide shapes and thepercent conversion to basic zinc carbonate of such shapes undersubstantially identical carbonation conditions are given.

                  TABLE II                                                        ______________________________________                                        Bulk Density                                                                  Starting ZnO Shapes                                                                           % Conversion ZnO to                                           (lb. per cu. ft.)                                                                             Basic Zinc Carbonate                                          ______________________________________                                        47              76                                                            56              63                                                            59              66                                                            62              55                                                            71              36                                                            72              33                                                            84              31                                                            ______________________________________                                    

While shapes of low bulk density allow for higher conversions to basiczinc carbonate, such shapes do not generally possess adequate strength.Additionally, the bulk density of the final product is directly relatedto that of the starting shapes in that an increase of about 5 to about15% is realized in the course of the process of the present invention.However, since the degree of conversion varies inversely with bulkdensity and since surface area as seen in Table I is dependent on thedegree of conversion, an upper limit in bulk density of about 90 lb. percubic foot is apparent from the data in Table II. Similarly, it ispreferred that the resultant zinc oxide product should have a bulkdensity of at least about 50 pounds per cubic foot. The preferredstarting range is about 55 to 65 pounds per cubic foot which results ina bulk density in the order of about 60 to 70 pounds per cubic foot forthe product.

The conditions under which the carbonation reaction is effected must besuch that basic zinc carbonate (5ZnO·2CO₂ ·3H₂ O) rather than normalzinc carbonate (ZnCO₃) is formed. The latter results in products of lowsurface areas probably due, at least in part, to the higher temperaturesrequired to convert it to zinc oxide. Among the reaction conditionsfound suitable for the formation of basic zinc carbonate are a flow ofcarbon dioxide at about normal pressure, temperatures of about 50° to90° C and reaction time of about 1 to 4 hours. Conversion temperaturesin excess of about 75° C result in an appreciable decrease in the rateof the reaction and should be avoided. Basic zinc carbonate obviouslyrequires water and in that regard, the effect of water vaporconcentration in the carbon dioxide upon the conversion of zinc oxide tobasic zinc carbonate is presented in Table III, wherein it will be seenthat desired levels of conversion starting with dry shapes, can beattained provided sufficient moisture is available in the carbon dioxideatmosphere. However, the levels of moisture in the carbon dioxideatmosphere required with dry shapes result in problems with condensationin the carbonation reactor. Additionally, forming normal zinc oxide intoshapes of a desired configuration is more easily effected with additionof some water.

                  TABLE III                                                       ______________________________________                                        Bulk Density                                                                            % H.sub.2 O                                                                           Carbonation                                                 ZnO Shapes*                                                                             in      Atmosphere  % Conversion ZnO to                             (lbs. per cu. ft.)                                                                      Shapes  % H.sub.2 O                                                                           % CO.sub.2                                                                          Basic Zinc Carbonate                          ______________________________________                                        56        0       50      50    62                                            53        8       --      100   30                                            53        8       22      78    64                                            62        14      --      100   46                                            60        17      --      100   60                                            57        23      --      100   73                                            59        22      20      80    74                                            50        29      --      100   69                                            50        29      20      80    50                                            ______________________________________                                         *Pellet shapes dried before bulk density determination.                  

Under the above circumstances, shapes containing in excess of about 15%moisture are preferred. The moisture content should not be such that thephysical properties of the shapes are impaired. In the event that therequired amount of moisture cannot be incorporated into the normal zincoxide during the shaping operation, additional moisture may beintroduced onto the shapes by spraying. In one such case, shapescontaining about 8% moisture were sprayed with water to increase themoisture content to about 22%; the degree of conversion to basic zinccarbonate in dry CO₂ after 2 hours was thereby increased from about 30%to slightly above 70%.

The conditions under which the decomposition of the basic zinc carbonateis effected are also of importance. In order to effect the conversion tozinc oxide in a reasonable time, i.e., about 1 to 2 hours, temperaturesof at least about 300° to 350° C have been found necessary. In Table IVthe effect of temperature upon the surface area of shapes that had been69% converted to basic zinc carbonate is set forth.

                  TABLE IV                                                        ______________________________________                                                 % Loss      Surface Area                                             Temp. ° C                                                                       in Weight   (square meters per gram)                                 ______________________________________                                        325      19.6        53                                                       350      19.4        44                                                       375      19.6        36                                                       400      19.8        32                                                       ______________________________________                                    

The weight losses indicate that the decomposition of the basic zinccarbonate was substantially complete in each case. As temperatureincreased, the surface area decreased significantly. Excessivetemperature can seriously affect not only the increase in surface areabut also the crushing strength of the shapes produced according to thisinvention as can be seen in Table V.

                  TABLE V                                                         ______________________________________                                        Temperature*                                                                            Average Crushing                                                                            Surface Area                                          (° C)                                                                            Strength** (gm)                                                                             (square meters per gram)                              ______________________________________                                        300       2710          44                                                    350       1910          35                                                    400       1720          24                                                    450       1710          20                                                    500       1580          16                                                    550       1090          11                                                    600        400           7                                                    ______________________________________                                         *The samples were heated for 5 hours at the indicated temperature. This       was to accelerate the affect of temperature over the preferred 1 to 2         hours shown in Table IV.                                                      **The method used to determine the crushing strength of individual shapes     is described in the following example. Ten trials were used to calculate      an average crushing strength.                                            

EXAMPLE

Approximately 40 lb. of an American process normal zinc oxide analyzingabout 99% ZnO and having a surface area of about 4 square meters pergram was formed into -4, +6 mesh pellets on a 39 inch diameterpelletizing disc. The pelleting conditions were:

    ______________________________________                                        ZnO Feed Rate                                                                             --     Approx.     1.6 lb./min.                                   Water Spray --     "           0.37 lb./min.                                  Disc Speed  --     "           17 rpm.                                        Disc Angle  --     "           55 to 60°                               ______________________________________                                    

The pellets containing 17% H₂ O and had a moist bulk density of 72 lb.per cubic foot (dry bulk density -- 60 lb. per cubic foot).

A portion of these pellets was treated with 4 liters per minute of CO₂for three hours in a 4 inch i.d. ×24 inch long porcelain thimble,preheated to 60° C in a rotary furnace operated at 1 rpm. Within 20minutes, the temperature rose to 80° C, indicating a rapid conversion ofthe zinc oxide to basic zinc carbonate, an exothermic reaction.Thereafter, for the next 55 minutes, the temperature slowly decreased toabout 60° C and for the remainder of the run was maintained betweenabout 60° C and about 70° C. At the end of 3 hours, conversion of thenormal zinc oxide to basic zinc carbonate was about 52%. To effectdecomposition of the basic zinc carbonate, the carbonated pellets wereheated in a tray in a muffle furnace for about 2 hours at 325° to 350°with an air flow of about 2 liters per minute. The surface area of thefinal product was 30 square meters per gram; bulk density was 64 lb. percubic foot.

The crushing strength of the pellets at various stages in the processwas determined by placing a single pellet between a rigid, stationary,metal rod and the pan of a double-beam balance. An upward force wasapplied to the bottom of the pellet by adding weights to the oppositepan until the pellet was crushed against the metal rod. The results of10 such trials for each type of pellet are recorded in Table VI. It willbe noted that the carbonated pellets exhibit a crushing strength about40 times greater than that of the starting pellets and that 70 to 80% ofthat strength is lost when decomposition of the basic zinc carbonate iseffected, probably because of the increase in porosity occasioned by theevolution of carbon dioxide and water vapor. Nevertheless, the strengthof the product of this invention is still twice that of a presentcommercial product.

                  TABLE VI                                                        ______________________________________                                                             Crushing Strength                                                             (gm)                                                     Pellets at Process Stage Indicated                                                                 Average                                                  ______________________________________                                        1. ZnO Pellets from Disc (17% H.sub.2 O)                                                           150                                                      2. ZnO Pellets dried at 110° C                                                              310                                                      3. After 52% Conversion to BZC                                                                     5660                                                     4. After Decomposition of BZC                                                                      1620                                                     Commercially Available ZnO Pellets                                                                 820                                                      ______________________________________                                    

The effectiveness in sulfur removal of the product of the presentinvention was compared with that of a commercially available product andalso with that of uncarbonated zinc oxide pellets by passing a drynitrogen gas stream into which hydrogen sulfide was admixed throughidentical beds of each type of pellet. In order to reduce the timerequired for these tests, the hydrogen sulfide concentration was set at15,000 ppm, bed temperature was 340° C, and the gas flowed at a spacevelocity of 1260 hr.⁻¹. The effluent gas was monitored for hydrogensulfide. When the hydrogen sulfide concentration in the effluent gasexceeded 1 ppm, the pellet bed was considered to have ceased to functioneffectively. In Table VII wherein the results of these tests arerecorded, it will be seen that the product of this invention is superiornot only to normal ZnO pellets but also to commercial material presentlyfor this purpose.

                  TABLE VII                                                       ______________________________________                                                        Surface  Bulk     Time to                                                     Area     Density  >1 ppm H.sub.2 S                                            (sq.m.   (lb./    in Effluent                                 Description     /gm.)    cu.ft.)  Gas (hr.)                                   ______________________________________                                        Normal ZnO Pellets                                                                            5        60       0.5                                         ZnO Product This Invention                                                                    31       64       3.7                                         Commercial ZnO Product                                                                        33       65       2.1                                         ______________________________________                                    

In fact, the product of the present invention is almost twice aseffective as the commercially available product, a difference far inexcess of that which might be expected from the fact that the ZnOcontent of the commercial product is about 85% as compared to about 99%ZnO for the product of the present invention.

Although specific embodiments of the invention have been described andshown, it is to be understood that they are meant to be illustrativeonly and not limiting. Certain features may be changed without departingfrom the spirit or essence of the invention. It is apparent that thepresent invention has broad application. Accordingly, the invention isnot to be construed as limited to the specific embodiments illustratedbut only as defined in the following claims:

What is claimed is:
 1. A process for producing high surface area andhigh crush strength particulate zinc oxide shapes, said processcomprising:a. forming normal zinc oxide having a specific surface arealess than 10 square meters per gram into pre-determined shapes withoutthe addition of extraneous binding material; b. exposing said normalzinc oxide shapes to a carbon dioxide-containing atmosphere includingwater vapor; c. converting at least about 45% of said zinc oxide intobasic zinc carbonate; and d. decomposing substantially all of said basiczinc carbonate into particulate zinc oxide shapes.
 2. The processaccording to claim 1 wherein said normal zinc oxide shapes contain up toabout 30% water.
 3. The process according to claim 1 wherein said normalzinc oxide shapes have a bulk density between about 45 and about 85 lb.per cubic foot.
 4. The process according to claim 1 wherein said normalzinc oxide shapes are exposed to said carbon dioxide-containingatmosphere at temperatures between about 50° and about 90° C for betweenabout 1 and about 4 hours.
 5. The process according to claim 1 whereinsaid carbon dioxide-containing atmosphere comprises at least about 60%carbon dioxide.
 6. The process according to claim 1 wherein said carbondioxide-containing atmosphere comprises between about 90 and 100% carbondioxide and between about 0 and about 10% water vapor.
 7. The processaccording to claim 1 wherein said basic zinc carbonate is decomposed ata temperature between about 200° and 500° C.
 8. The process according toclaim 1 wherein said basic zinc carbonate is decomposed at a temperaturebetween about 250° and about 350° C.
 9. The process of claim 1 whereinsaid particulate zinc oxide shapes have a crushing strength betweenabout five and about thirty times greater than the crushing strength ofthe predetermined zinc oxide shapes.